1. Field of the Invention
The present invention relates to an electronic flash unit having an insulated gate bipolar transistor (IGBT) which is connected in series with a flash tube and serves to control the light-emission from the tube, and more particularly to a flash unit having a simiplified drive control system for the IGBT.
2. Description of the Related Art
One of previously known flash units provided with the above IGBT is disclosed in U.S. Pat. No. 4,839,686.
As shown in FIG. 8, this flash unit is composed of a DC high voltage power source 1 which is a known DC--DC converter circuit, a main capacitor 2 which will be charged by the power source 1, a constant voltage circuit 3, attached to the power source 1, for supplying a constant voltage to a light-emission control circuit 7 described later, a known trigger circuit 4 for triggering the flash tube 5, a control circuit 6 which is connected with control means 8 incorporated in a camera body so that several kinds of signals are transferred between them thereby producing several kinds of output signals such as a trigger signal for operating the trigger circuit 4, a light-emission control circuit 7 for on-off controlling an IGBT connected in series with the flash tube 5 to control the light-emission from the flash tube 5, and a double voltage circuit 9 for applying a doubled voltage to the flash tube 5.
In operation, when a switch SW is switched on, the DC high voltage power source 1 operates to charge the main capacitor 2 and a double voltage capacitor 9a in their polarity indicated. At the same time, a DC low voltage power source E charges a power supply capacitor C for the control circuit 6, and the above DC high voltage power source also charges a capacitor 3a incorporated in the constant voltage source 3.
If with each of these capacitors charged, the control means 8 supplies a light-emission starting command signal to the control circuit 6, the control circuit 6 will produce, from its one output terminal Oa, a trigger signal at a high level for a predetermined period in which the longest light emitting period of the flash tube 5 is considered. And when the light-emission starting command signal is supplied, the other output terminal Ob of the control circuit 6 is held at a low level so that a transistor Qc remains off. Thus, transistors Qa and Qb turn on, and the charging voltage of the capacitor 3a is applied to the gate of the IGBT so that the IGBT turns on.
When the IGBT turns on, the known trigger circuit 4 operates to excite the flash tube 5 and also the (+) side of the double voltage capacitor 9a is grounded through a resistor R1 and the IGBT; the charged energy of the double voltage capacitor 9a is superposed on that of the main capacitor 2 so that the charged energy thus prepared will be supplied to the flash tube 5. Accordingly, the flash tube 5 emits light by consuming the charged energy in the main capacitor 2.
If in the course of emitting light, a photometer circuit incorporated in the control means 8, for example, supplies a light-emission stopping command pulse to the control circuit 6, the control circuit 6 will produce a high level light-emission stopping signal from its output terminal Ob, and the transistors Qc and Qd turn on. Thus, the transistor Qa is short-circuited between its base and emitter and the IGBT is short-circuited in its gate and emitter so that these transistors turn off. Then, the transistor Qb also turns off and the flash tube 5 stops emitting light.
The operation described above is a basic operation of the flash unit as shown in FIG. 8. This flash unit can obviate excess of light-emission in contrast to the flash unit which stops emitting light using a terminating capacitor, and can repeatedly emit light at a high speed.
However, the conventional flash unit as shown in FIG. 8 has the following disadvantages.
The system of driving the IGBT operates in response to both a trigger signal and a light-emission stopping signal so that the means 7 for controlling the voltage supply to the gate of the IGBT is required. Namely, as seen from FIG. 8, the control switch arrangement composed of the transistors Qa to Qc, etc. is required. This results in a complicated circuit construction of the flash unit, which leads to high production cost.
Further, since the trigger circuit 4 starts to operate in response to the trigger signal, and simultaneously the charging voltage is applied to the gate of the IGBT, the trigger circuit 4 may operate before the IGBT has fully turned on. Then, the IGBT is in a high impedance state; this deteriorates the operation efficiency of the trigger circuit 4 so that the flash tube 5 may fail to emit light. Even if the flash tube 5 can emit light, the energy (charged energy) supplied from the main capacitor may destroy the IGBT. In short, in some cases, the operating timing of the trigger circuit 4 provides inconveniences of lowering the operation efficiency and braking the IGBT.